Mosquito borne pathogens are important causes of disease in both humans and animals. Historically, attempts to disrupt pathogen transmission cycles have depended upon reducing vector populations through the use of insecticides or immunization of vertebrate hosts. Unfortunately, development of insecticide resistance and the collapse of vector control programs necessitate the development of alternate strategies for the control of vector borne pathogens. Novel approaches aimed at reducing vectorial capacity by elucidating elements of virus/vector interactions at the molecular level may be one of the most effective methods in attaining this goal and thus, controlling the spread of arthropod-borne viral disease. Alphaviruses are one of the best studied genera of arthropod borne viruses. Because the genome of these viruses consists of a single stranded, positive sense RNA molecule and many alphaviruses are well characterized at the molecular level, they are ideally suited for studies designed to elucidate mechanisms of interaction with their mosquito vectors. This proposal focuses on the alphaviruses o'nyong nyong (ONN) and chikungunya (CHIK). ONN is unique among arthropod-borne viruses in that it is transmitted by Anopheles species mosquitoes. In contrast, CHIK, the virus most closely related to ONN, is transmitted by Aedes mosquitoes. Using infectious cDNA clone technology, the genetic differences between these two viruses allowing transmission by such disparate vector hosts can be examined. The specific aims of this project are to: (1) Evaluate the differential infectivities of CHIK and ONN viruses in Anopheles gambiae mosquitoes to contrast the patterns of infection, replication, and dissemination within an epidemiologically significant vector. This will provide information on the biological differences that exist between closely related viruses maintaining distinct cycles of transmission; (2) Construct chimeric infectious clones containing portions of both ONN and a closely related African strain of CHIK virus. These parental and chimeric clones will be used to test the central hypothesis that specific components of the ONN viral genome are responsible for the unique ability of this virus to infect and be transmitted by Anopheles species mosquitoes; (3) Perform adaptation studies using an infectious clone of CHIK virus to determine the potential for development of mutations that lead to changes in cell and vector specificity and virulence patterns. The molecular dynamics involved in virus interactions with their mosquito vectors have gone largely unstudied. Developing a greater understanding of these interactions will provide tools for investigating the molecular biology of epidemiologically important disease vectors as well as aid in the design of potential new targets for the control of vector borne diseases.